Subsea installations are assemblies used under water. Such assemblies can be installed, for example, on the bottom ground of a body of water. Power transformers are an example of subsea installations which can be used under water. Generally, power transformers use an insulation medium. For example, the power transformers can be fluid insulated having transformer oil in a transformer tank. Examples of other subsea liquid filled objects are subsea motors, subsea switchgears, subsea converters, rectifiers and hydraulic store tanks.
The subsea installations described above are used, for example, in modern oil and gas production in which the collection, separation, boosting and transport of production fluids takes place on the seabed. These processes require large amounts of power that has to be transferred from a remote location at suitable voltages and currents to achieve minimum power loss, to the subsea installations. The transport of the power can take place at high voltages and low current to ensure minimum losses. When the power arrives at the subsea location, the power has to be transformed into more usable voltages and currents for the various specific subsea applications, such as the powering of pumps and compressors, for example.
The large power requirements result in the need for large transformers to be placed in a protective environment inside a large vessel, e.g. a tank. The transformer tank is filled with an insulating medium that ensures optimal working conditions for the transformer over many years. Transformer oil that contributes towards dissipating heat and preventing shorts and flashovers is a common example of such an insulating medium. In new big subsea transformers, the size of the transformer tank can be in the order of 10-30 cubic meters containing 10,000 to 30,000 litres of transformer oil. The water temperature variations, heat produced by the transformer and the properties of transformer oil result in oil volume variation in the order of several hundred litres or more. As the size of a transformer tank increases, the problems and need for compression and expansion also increase accordingly.
Subsea installations such as subsea transformers need pressure compensators to keep the pressure of the insulation medium used, for example the pressure of oil inside the transformer, close to the water pressure outside the transformer to avoid heavy mechanical structures against pressure. Also, cooling is easier when thinner wall thickness can be applied. The hydrostatic pressure of the water will increase circa 10 bars for each 100 m water depth increase, and the pressure difference between the oil inside and the water outside should typically be less than 1 bar. The subsea transformers filled with insulation oil will be exposed to oil compression and expansion due to temperature variation of the surrounding, due to load variation also causing a variation of the oil temperature and due to hydrostatic pressure of the sea water. The oil volume variation due to temperature variation is caused by a property of the oil having a non-zero thermal expansion coefficient, and the oil volume variation due to pressure variation is caused by close to zero pressure-volume coefficient of the oil.
From the point of view of construction and reliability, the transformer vessels or other equipment tanks described above are known to be rigid and constant volume structures. When transformer or equipment sizes are increasing, this means that the pressure inside the transformer or other equipment must be close to water pressure around the transformer or the other equipment. Owing to the factors above, the subsea transformers or the other equipment are equipped with one or several pressure compensators to keep the oil pressure inside the transformer close to the water pressure outside the transformer. The pressure compensators are structures with variable volume oil or other liquid filled containers outside the transformer or the other equipment. The variable volume containers are also in flow connection with the transformer oil space or the other equipment liquid space.
There are several different solutions for subsea pressure compensators. WO 2007/055588 A1 and U.S. Patent Application US 2004/0051615 A1 are examples of known solutions. All known subsea pressure compensators, such as those mentioned above, for example, have only one wall or barrier between the sea water and the inside of the oil volume. This means that if one barrier or wall will have a failure producing leakage between the oil and the water, the water will enter into the transformer causing electrical failure of the transformer and thereby affect the operation of the transformer. The other equipment will have also serious consequences such as electrical failures if the water will leak in. Such leak problems relate mainly to corrosion caused by the sea water, because known pressure compensators are not particularly resistant against corrosion caused by sea water. The sensitivity of the corrosion may be a risk with metallic bellows type pressure compensators because the wall of the bellows should be very thin to reach required flexibility. This may be the situation even when cathodic protection will be used.
There are also known solutions with which properties of different bellows structures against corrosion are improved. JP 9176766, JP 2000046181, EP 0 281 685 and U.S. Pat. No. 5,893,681 are examples of such solutions. These known solutions are not, however, suitable in subsea conditions.